The present invention relates to a bipolar ionic current probe unit adapted to measure a positive ionic current density and a negative ionic current density and a method for measuring positive and negative ionic current densities by means of the same bipolar ionic current probe unit.
Heretofore, in an electric dust-collecting apparatus for collecting dust having an extremely high electric resistance, after a voltage drop across a dust layer accumulated on a dust-collecting electrode would become excessively large, resulting in dielectric breakdown, and thus the so-called back ionization would occur, which causes abnormal positive corona discharge from the dust-collecting electrode towards a discharge electrode of negative polarity. This served as a principal factor for greatly degrading the dust-collecting performance of the apparatus. This degradation of the dust-collecting performance is caused by the fact that negative charge on the dust which was applied by bombardment by a negative ionic current produced by negative corona discharge of the discharge electrode is neutralized and reduced by a positive ion current fed by the above-mentioned anomalous positive corona discharge and consequently a Coulomb's force necessitated for collecting dust is greatly weakened. In this instance, even when the ratio .beta.=i.sub.+ /i.sub.- of a positive ionic current density i.sub.+ to a negative ionic current density i.sub.- is as small as only 10%, a quantity of electric charge Q on the dust is reduced to 50% with respect to its ideal value Q.sub.o which can be attained by negative ions fed from the discharge electrode upon normal operation when no back ionization occurs, and as the value of the ratio .beta. is increased successively to 20%, 30% and 40%, the quantity of electric charge Q is widely reduced to 30%, 20% and 10%, respectively, of the ideal value Q.sub.o, so that remarkable degradation of the dust-collecting performance is resulted to the corresponding extent. In order to investigate such anomalous phenomena, to confirm the extent of degradation and to appropriately control voltages, currents and the like in a dust-collecting apparatus on the basis of the results of investigation and confirmation, it is necessary to measure a positive ionic current density i.sub.+ and a negative ionic current density i.sub.- individually and separately within an electric field in which both positive and negative ionic currents coexist. However, in the prior art such separate measurement was considered to be impossible, and a big problem of achieving such separate measurement of positive and negative ionic current densities has been left unsolved up to the present.